Dear authors, I have read your paper with interest, and I have a couple of remarks that I think needs to be discussed to hopefully stimulate to more studies in this important field.
It is a weakness that there are very few randomised trials on surgical treatment of tendinopathy. To then draw conclusions from few studies is difficult and not always possible.
From this systematic review it is shown that 10 out 12 trials are on tendinopathy in the upper extremities (shoulder and elbow-althogether 998 tendinopathies) and only 2 on tendinopathy in the lower extremities (patellar and Achilles-althogeteher 60 tendinopathies). The authors have unfortunately missed to include 2 randomised studies on surgical treatment of patellar tendinopathy, published by Dr Willberg et al. Althogether, most information is about the shoulder and elbow, and very little about the Achilles and patellar tendons. Anyhow, when conclusions are drawn I get the impression that the authors put all tendinopathies in one group. This might be strongly misleading since there is very little information about the lower extremity tendons (only 2 studies), and conclusions cannot be drawn for Achilles and patellar tendinopathy. Furthermore, such conclusions might lead to that we miss possible differences in load response between upper and lower extremity tendons? The upper extremity non weight-bearing tendons might respond different compared to the lower extremity weight bearing tendons, as it is for skeletal bones. Therefore, until we have more knowledge, I suggest that conclusions about tendinopathies are separated to upper and lower extremity.
-The conclusion in the abstract that tendinopathies should be treated with physiotherapy for at least 12 months before the option of surgery is seriously entertained cannot be stated based on the material in this systematic review. Although I myself is a strong promoter of exercise treatment for tendinopathy, at least for the Achilles and patellar tendons there is not enough research backing up such a statement.
-The conclusion that surgery is not superior to sham surgery in patients with tendinopathy is not correct-this can maybe be indicated considering the shoulder and elbow (based on very few studies), but for the lower extremity there are no such studies to refer to.
Althogether, this review clearly shows that much more research is needed in this field.

The article Mortality of Japanese Olympic athletes: 1952 – 2017 cohort study currently fails to account for a probable source of non-negligible bias. Whilst the main finding, stating that there is a decreased mortality rate among Japanese Olympians appears to be methodologically reliable, the authors also state that “higher mortality was observed among those who participated in the Olympics twice and three times or more compared with those who participated just once,” which we believe to be a potentially inaccurate finding.
A delayed entry exists at baseline, as those with longer careers tend to be older than those with shorter careers. For instance, someone who has participated in 3 Olympic Games started her/his Olympic career 12 years before those who have participated only once. Thus, there might be an important period effect underlying these findings. Even if the authors have adjusted the analysis by age, this is not equivalent to adjusting for period, as earlier periods in time are related with higher mortality rates, which could explain the findings.
Additionally, another problem often seen in survival studies is ignoring when a death event occurs. This is problematic because deaths will be observed more frequently in subjects with longer follow-up times. In the presence of time-dependent bias, the hazard ratio is artificially underestimated and the length bias leads to an artificial underestimation of the overall hazard [2]. Therefore, faulty interpretation may result from the underlying assumptions, which is potentially the case in this study.
The authors argue that the higher mortality rate found among those who participated in multiple Olympics could be related to longer periods of intense training. Yet, this is contradictory to their main finding which demonstrates far lower mortality rates among Olympians in comparison to the general population.
Finally, the higher mortality found among those engaged in multiple Games could also be explained by the missing life status confirmations. There were 1324 Olympians who were assumed to be alive when missing date of death. Previous studies that dealt with the same data source and the submission of life status to national data certification for confirmation found that there was more reliable data for well-known athletes [3]. Thus, date of death is probably more widespread for athletes engaged in multiple games and therefore less well-known athletes, participating in only one Games, may have been mislabeled alive when death information was missing.
The main findings that demonstrate lower mortality rates among Japanese Olympians compared to the general population rely on standard methods and seem to be reliable. The secondary findings that compare mortality among Olympians engaged in multiple games should be revised accounting for the biases described in this letter.

1 Zwiener I, Blettner M, Hommel G. Survival analysis: part 15 of a series on evaluation of scientific publications. Dtsch Ärztebl Int 2011;108:163–9. doi:10.3238/arztebl.2010.0163
2 Wolkewitz M, Allignol A, Harbarth S, et al. Time-dependent study entries and exposures in cohort studies can easily be sources of different and avoidable types of bias. J Clin Epidemiol 2012;65:1171–80. doi:10.1016/j.jclinepi.2012.04.008
3 Antero-Jacquemin J, Pohar-Perme M, Rey G, et al. The heart of the matter: years-saved from cardiovascular and cancer deaths in an elite athlete cohort with over a century of follow-up. Eur J Epidemiol Published Online First: 5 May 2018. doi:10.1007/s10654-018-0401-0

To the Editor,

We read the recent publication by Nathan, Davies & Swaine (2018) with great interest due to a mutual interest in the subject of Generalised Joint Hypermobility (GJH) and its influence on injuries within elite sport. The authors of this paper should be commended for undertaking a study with such good participant numbers over a range of sports. We believe that the findings of this study suggesting that GJH may be protective of joint ligament damage may be a very important initial paper leading to valuable further exploration within specific sports and specific joints. However despite this good work we would like to take the opportunity to raise a concern over one of their conclusions and how this may confuse readers of the article.

In the discussion section of this paper Nathan et al. (2018) suggest that the findings of this study may suggest that “regular stretching may increase flexibility, and this could subsequently reduce rates of injury in those that are less flexible.” We believe that this statement may lead to misunderstanding as the terms “flexibility” and “joint hypermobility” are two completely different entities.

GJH is a hereditary physiological entity whereby most synovial joints move beyond their normal limits (Pacey et al., 2010) and may, or may not be symptomatic. This entity is commonly classified by the use of the Beighton Scale, as in the Nathan et al. (2018) paper, whereby adult participants are deemed positive if they score five or more out of nine on the scale (Juul-Kristensen et al., 2017). The term flexibility however would commonly be recognised as the length of the muscle crossing the joint. It is suggested that within the paper the term “flexible joint” is overused, and the correct terminology should be “lax joint” as this would more closely match the fact that the authors are looking to further understand the influence of joint hypermobility.

We would suggest concluding that greater flexibility would decrease injury risk means the authors are misleading readers. It would be hypothesised that the decreased risk of injury within the ankle for example, may come from the ability of the athlete to enter ranges of inversion without stressing the passive ligamentous structures of the ankle which is thought to be in part due to the abnormal ratio of mobile type III collagen in hypermobile participants (Child, 1986). Flexibility of the muscles around the ankle joint is unlikely to provide a protective effect in this position. It is also worth noting that the terminology in the paper suggests that rates of joint sprains are lower in hypermobile participants highlights the fact that the authors were looking at ligamentous injury, and not muscular pathologies. It is unlikely that a stretching protocol would be able to influence the mobility of the passive ligamentous structures to such a degree that they would subsequently be classed as hypermobile, and therefore potentially at decreased risk of injury.

We would again like to highlight that this piece of work by Nathan et al. (2018) is a very important piece in the early stages are trying to better understand the influence of GJH upon injury risk. It is hoped that greater utilisation of the Lower Limb Assessment Score in sports where lower limb pathologies are most common may be able to assist in addressing the impact on practice suggestions from the authors.

References
Child, A.H. (1986). Joint hypermobility syndrome: Inherited disorder of collagen synthesis. The Journal of Rheumatology, 13(2), 239-243.

Juul-Kristensen, B., Schmedling, K., Rombaut, L., Lund, H. & Engelbert, R.H.H. (2017). Measurement properties of clinical assessment methods for classifying generalized joint hypermobility- A systematic review. American Journal of Medical Genetics Part C (Seminars in Medical Genetics), 175C, 116-147.

Nathan, J.A., Davies, K. & Swaine, I. (2018). Hypermobility and sports injury. BMJ Open Sport & Exercise Medicine, 4.

Pacey, V., Nicholson, L.L., Adams, R.D., Munn, J. & Munns, C.F. (2010). Generalized Joint Hypermobility and Risk of Lower Limb Joint Injury During Sport: A Systematic Review with Meta-Analysis. The American Journal of Sports Medicine, 38(7), 1487-1497.

Dear Editor in Chief:

We read with great interest the recently published article by Tibana and de Sousa (1) titled “Are extreme conditioning programmes effective and safe? A narrative review of high intensity functional training methods research paradigms and findings.” We appreciate the opportunity to write this letter and hope to clarify some of the authors’ conclusions. Although the authors provide several examples of what they refer to as “extreme conditioning programs” we will focus mainly on the statements and evidence related to High Intensity Functional Training (HIFT), more commonly known as CrossFitTM training, as the authors’ review focuses primarily on this particular training program. We feel the authors have taken a biased position in describing this type of training and that their position is based on inaccurate and highly speculative interpretations of a fraction of the existing literature.

Research examining the acute and long-term responses to HIFT, as well as the incidence of injury, is quite limited. The observed responses predominantly describe changes from baseline and in the case of long-term adaptations, generally show a positive outcome. Further, the few studies that make comparisons to other exercise forms only show select differences. More importantly, by the authors’ own admission, research examining the risk of injury do not suggest HIFT/CrossFitTM to be different from other forms of recreational exercise. Yet, the authors describe this risk as “alarming” and “becoming more prevalent.” We certainly agree that additional research is needed on this popular form of exercise, but at this time, and based on the available evidence, any definitive conclusions are premature at best.

From their terminology to generalized conclusions, it is clear that Tibana and de Sousa (1) have taken a position on HIFT/CrossFitTM that fits the definition of “extreme”. However, the Merriam-Webster dictionary describes extreme as something “existing in a very high degree,” or “exceeding the ordinary, usual, or expected” or even “relating to, or being an activity or a form of a sport (such as skiing) that involves an unusually high degree of physical risk” (2). Yet, the scientific literature around HIFT/CrossFitTM does not support this notion or the use of “extreme” as an accurate descriptor. More importantly, the authors seem to have misrepresented some of the data. For example, alternating sets of push-ups and pull-ups on each minute for 48 minutes and totaling 400 push-ups and 200 pull-ups is equivalent to completing approximately ~16 push-ups and ~8 pull-ups per minute. Although this might appear to be a difficult task to some, it is not made clear by Tibana and de Sousa (or the original authors) how these tasks were performed (e.g., push-up starting position; assisted, butterfly, or kipping pull-ups) which would impact the severity of the exercise (3, 4). More importantly, the authors failed to mention that the individual played ice hockey on the following night and participated in another high-intensity workout the day after before seeing a medical professional. The authors also described additional protocols (e.g., 105 push-ups in 6 minutes [or 17 push-ups per minute]; 3 sets of chin-ups to failure over a 20-minute period) as being “extreme” when they were generally in line with the recommended prescription for endurance exercise (5). Another example of misrepresentation occurred with their description of the ‘Sissy Test’. This workout consists of 240 total repetitions evenly split between kettlebell swings and burpees, not 672 total repetitions. Likewise, the competition workout that included “unaccustomed exercise” (i.e., 60 GHD’s) is simply a version of the sit-up exercise performed with greater range of motion. In short, if we are to accurately describe HIFT and assess its merits and limitations, it is important to maintain perspective, describe protocols in detail, and avoid exaggerations.

Feito et al. (6) provided a working definition for HIFT: “a training style [or program] that incorporates a variety of functional movements, performed at high-intensity [relative to an individual’s ability], and designed to improve parameters of general physical fitness (e.g., cardiovascular endurance, strength, body composition, flexibility, etc.) and performance (e.g., agility, speed, power, strength, etc.).” Unlike “Extreme Conditioning Programs,” which would suggest that training is “abnormal” or “inherently dangerous,” this definition provides an objective description of workout modality and intensity, as well as its targeted outcomes. We acknowledge that this particular definition may not have been available at the time of the authors’ manuscript submission, but it is not drastically different than the definition provided by CrossFitTM itself: “constantly varied, high-intensity, functional movement” (7). Further, the term “HIFT” was first introduced in 2014, when Heinrich and colleagues (8) compared high-versus moderate-intensity training programs and has been used in more than a dozen other studies since. Tibani and de Sousa use ‘HIFT’ within their title, but the term is otherwise absent throughout their manuscript. We believe this omission, along with their exclusion of other works, represents a considerable amount of bias and limits the ability of the authors (and readers) to form accurate, objective conclusions on the impact of HIFT. Moreover, several studies have provided insight to the cardiovascular responses of HIFT training (9-11) and suggest these programs fit within the current exercise prescription recommendations provided by the American College of Sports Medicine (12).

Regarding injuries related to HIFT, the authors acknowledge that the incidence of injury (approximately 2 – 4 injuries per 1000 hours) is similar to that of weightlifting and other recreational activities and lower than several contact sports. Though limited by the cross-sectional nature and small sample sizes, these rates are consistent with what others have reported (13-19). As such, we agree with the authors that additional studies are needed to elaborate on the potential risk of this training modality, particularly among different population subgroups (i.e. youth, older adults, etc.). However, we do diverge from the authors’ presentation of this information throughout the manuscript. The authors introduce three areas of concern that are related to participant’s safety: “(1) use of fast movements with a high number of repetitions and insufficient rest intervals; (2) participants with a weak motor repertoire to perform complex movements and (3) a possible lack of educated, certified and experienced ECPs professionals, especially in the integration of all training methods.” Unfortunately, the authors do not provide evidence to validate these concerns.

Tibana and de Sousa cite a pair of studies that indicate a greater occurrence of overuse injuries (20) and a higher prevalence of shoulder injuries (14) following HIFT, but neither of these studies report the programming characteristics (i.e., training volume, relative intensity loads, or exercise frequency) employed by their participants. To make generalizable conclusions such as these, these authors (14, 20), as well as Tibani and de Sousa, must assume that higher training volumes, high training intensities, and complex movements are consistently employed across training. If we are to assume that HIFT/CrossFitTM is being appropriately employed, then this cannot be the case. By definition (6, 7), HIFT is constantly varied and scalable (i.e., intensity is relative to the individual), and would suggest that participants do not regularly employ any specific volume, intensity or modality with sufficient regularity to evoke overuse injuries. Nevertheless, without an accurate programming record, it is impossible to conclude that HIFT/CrossFitTM participants are regularly exposed to inappropriate complex movements that lead to injury. Likewise, it is not suitable to assume HIFT participants possess a “weak motor repertoire to perform complex movements” or that coaches lack education, certification, or experience. Although not provided by the authors, evidence refuting these claims have been provided by Tafuri et al. (21) and Waryasz et al (22), respectively.

Another important topic related to injury discussed by the authors is that of exertional rhabdomyolysis (ER). Regardless of the activity, developing ER is an extreme case that is detrimental to physical performance improvements and may lead to death. Nevertheless, the authors provide a very narrow view of how ER can develop and their conclusion that it is “becoming increasingly more prevalent” in HIFT is unsupported. The evidence provided within the manuscript consists of nine case studies that detail the occurrence of ER in 15 individuals. When put into perspective, the incidence of ER in relation to HIFT is quite small. Compared to the number of CrossFitTM affiliates existing worldwide (approximately 13,000), this would equate to 0.12% of participants if each location had a single member develop ER. Compared to the number of participants in the 2018 Open competition (N = 429,157 competitors), it would equate to less than 0.004%. A closer look at the literature related to ER demonstrates a similar number of case studies that support the development of ER in other activities (23-25), such as stationary cycling (26-28), ultimate frisbee (29) and even hiking (30).

In the most current and comprehensive study of HIFT related injuries, Feito et al. (19) demonstrated the overall low incidence of injuries associated with HIFT programs. According to that study, which included over 3,000 participants over a 4-year period, only 0.6% of participants reported the occurrence of ER (19). Even though the study by Feito and colleagues (19) is also limited in its ability to depict incidence of ER, it certainly provides a more appropriate measure of prevalence than any case study alone. Moreover, Knafl and colleagues (31) reported an increased number of ER cases being admitted to an emergency room in Brisbane, Australia and alluded to high-intensity workouts (e.g., CrossFitTM training) as a potential cause. However, this may have been merely speculation by the authors, as the underlying causes were divided into seven specific categories: 1) gym, 2) marathon/running, 3) manual labor, 4) military/police training, 5) team sports, 6) walking/heat, and 7) other (31). Therefore, we certainly agree that additional studies are warranted to accurately portray the incidence of ER, but we cannot yet conclude that HIFT presents any greater danger than other exercise forms.

In regards to the physiological responses to HIFT, Tibana and de Sousa refer to a number of studies documenting a larger inflammatory response compared to traditional exercise protocols. They summarize and interpret the literature as follows: “These results indicate that ECPs elicited a higher metabolic, cardiovascular, hormonal and inflammation response. Therefore, strength and conditioning professionals need to be aware of the level of stress imposed on individuals when performing metabolic workouts of ECPs… it is recommended that the incorporation of lower intensity sessions (e.g., through the rating of perceived exertion or HR) and/or resting days would help to minimize this exacerbated physiological response.” Although the authors partly summarize the more pronounced physiological responses, the recommendation to reduce intensity is not warranted, nor is the subjective description of the response as “exacerbated.” It is well documented that neuroendocrine and inflammatory responses play a critical role in exercise-induced signals for health-related benefits, including cardiovascular and metabolic adaptations (32, 33). It is further documented that these responses are directly correlated to the intensity of the exercise bout, and the degree of adaptations to exercise are, accordingly, primarily linked to the intensity of the exercise stimulus (34, 35). Therefore, we express the concern that the recommendation put forth by Tibana and de Sousa to reduce exercise intensity is tantamount to recommending a suboptimal exercise stimulus that would produce smaller magnitude health benefits compared to HIFT. Indeed, the authors failed to mention a very important recent study demonstrating that HIFT reduces fat mass, increases insulin sensitivity, increases fat oxidation, reduces blood concentrations of the pro-inflammatory cytokine resistin and increases levels of the anti-inflammatory cytokine adiponectin in people with type 2 diabetes (36).

With respect to the chronic adaptations associated with HIFT, Tibana and de Sousa reach the conclusion that the “scientific literature showed few or no chronic effects on body composition and improvements in physical fitness and psychological parameters; however further studies are important.” For two reasons, we fail to understand how the authors reached this particular conclusion. First, the evidence provided by the authors seems to contradict their conclusions. Namely, the majority (i.e., 6 out of 11 studies) of these studies demonstrate positive effects of HIFT interventions on both body composition and fitness outcomes. Second, the authors fail to include two recent (but available prior to the date of publication) investigations that further demonstrate these positive effects (37, 38). Thus, we would like to offer an alternative interpretation of the authors’ presented evidence with respect to the chronic adaptations to HIFT. We would argue that not only does the majority of the evidence support the efficacy of HIFT for multiple domains associated with health and fitness, it also demonstrates efficacy across multiple population subgroups (e.g., teens, cancer survivors, people with Type 2 Diabetes, etc.). Further, the considerable heterogeneity across these studies with respect to intervention dosing (e.g., included interventions ranged from five weeks to six months in duration) suggests the likelihood of a dose-response relationship between HIFT and its desired outcomes. Current evidence suggests that at least 15 HIFT sessions, over a 3 – 5-week period, are needed to elicit changes in fitness outcomes, but the minimum effective dose specific to body composition changes is not well established and could potentially be significantly higher than it is for fitness-related outcomes.

Overall, we commend Tibana and de Sousa’s for their work and their attempt to review the current available literature related to HIFT. However, based on our interpretation of the evidence presented herein, we urge caution on the part of the BMJ readership in regards to its presentation and applying the recommendations put forth in their article.

 
REFERENCES

1. Tibana RA, Sousa NMF. Are extreme conditioning programmes effective and safe? A narrative review of high intensity functional training methods research paradigms and findings. BMJ Open Sport & Exercise Medicine. 2018;0:e000435.
2. "extreme". Merriam-Webster.com [Available from: https://www.merriam-webster.com/dictionary/extreme.
3. Dinunzio C, Van Scoy J, Porter N, Cordice D, McCulloch R, editors. Kinematic and muscle activation differences between a standard pull-up and a dynamic CrossFit "kipping" pull-up. International Journal of Exercise Science: Conference Proceedings; 2017.
4. Ebben WP, Wurm B, VanderZanden TL, Spadavecchia ML, Durocher JJ, Bickham CT, et al. Kinetic analysis of several variations of push-ups. The Journal of Strength & Conditioning Research. 2011;25(10):2891-4.
5. Sheppard JM, Triplett NT. Program Design for Resistance Training. In: Haff GG, Triplett NT, editors. Essentials of Strength Training and Conditioning. 4th ed: Human Kinetics; 2015. p. 439 - 69.
6. Feito Y, Heinrich KM, Butcher SJ, Poston WSC. High-Intensity Functional Training (HIFT): Definition and Research Implications for Improved Fitness. Sports (Basel). 2018;6(3).
7. CrossFit Training. The CrossFit Level 1 Training Guide: CrossFit, Inc.; 2018. Available from: http://library.crossfit.com/free/pdf/CFJ_English_Level1_TrainingGuide.pdf.
8. Heinrich KM, Patel PM, O'Neal JL, Heinrich BS. High-intensity compared to moderate-intensity training for exercise initiation, enjoyment, adherence, and intentions: an intervention study. BMC Public Health. 2014;14(14):789-95.
9. Butcher SJ, Neyedly TJ, Horvey KJ, Benko CR. Do physiological measures predict selected CrossFit benchmark performance? Open Access J Sports Med. 2015;6:241-7.
10. Sperlich B, Wallmann-Sperlich B, Zinner C, Von Stauffenberg V, Losert H, Holmberg HC. Functional High-Intensity Circuit Training Improves Body Composition, Peak Oxygen Uptake, Strength, and Alters Certain Dimensions of Quality of Life in Overweight Women. Front Physiol. 2017;8:172-81.
11. Kliszczewicz B, John QC, Daniel BL, Gretchen OD, Michael ER, Kyle TJ. Acute exercise and oxidative stress: CrossFit™ vs. treadmill bout. Journal of human kinetics. 2015;47(1):81-90.
12. American College of Sports Medicine. General Principles of Exercise Prescription. In: Riebe D, editor. ACSM's guidelines for exercise testing and prescription. 10th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2017. p. 143-79.
13. Chachula LA, Cameron KL, Svoboda SJ. Association of Prior Injury With the Report of New Injuries Sustained During CrossFit Training. Athletic Training & Sports Health Care. 2016;8(1):28-34.
14. Hak PT, Hodzovic E, Hickey B. The nature and prevalence of injury during CrossFit training. J Strength Cond Res. 2013:Published Ahead of Print.
15. Mehrab M, de Vos R-J, Kraan GA, Mathijssen NM. Injury Incidence and Patterns Among Dutch CrossFit Athletes. Orthopaedic journal of sports medicine. 2017;5(12):2325967117745263.
16. Montalvo AM, Shaefer H, Rodriguez B, Li T, Epnere K, Myer GD. Retrospective Injury Epidemiology and Risk Factors for Injury in CrossFit. J Sports Sci Med. 2017;16(1):53-9.
17. Moran S, Booker H, Staines J, Williams S. Rates and risk factors of injury in CrossFit: a prospective cohort study. Journal of sports medicine and physical fitness. 2017;57(9):1147-53.
18. Weisenthal BM, Beck CA, Maloney MD, DeHaven KE, Giordano BD. Injury Rate and Patterns Among CrossFit Athletes. Orthopaedic Journal of Sports Medicine. 2014;2(4).
19. Feito Y, Burrows EK, Tabb LP. A 4-Year Analysis of the Incidence of Injuries Among CrossFit-Trained Participants. Orthopaedic Journal of Sports Medicine. 2018;6(10):2325967118803100.
20. Grier T, Canham-Chervak M, McNulty V, Jones BH. Extreme conditioning programs and injury risk in a US Army Brigade Combat Team. US Army Med Dep J. 2013(4):36-47.
21. Tafuri S, Notarnicola A, Monno A, Ferretti F, Moretti B. CrossFit athletes exhibit high symmetry of fundamental movement patterns. A cross-sectional study. Muscles, ligaments and tendons journal. 2016;6(1):157.
22. Waryasz GR, Suric V, Daniels AH, Gil JA, Eberson CP. CrossFit® instructor demographics and practice trends. Orthopedic reviews. 2016;8(4).
23. Gagliano M, Corona D, Giuffrida G, Giaquinta A, Tallarita T, Zerbo D, et al. Low-intensity body building exercise induced rhabdomyolysis: a case report. Cases J. 2009;2(1):7.
24. Galvez R, Stacy J, Howley A. Exertional rhabdomyolysis in seven division-1 swimming athletes. Clin J Sport Med. 2008;18(4):366-8.
25. Casares P, Marull J. Over a millon Creatine Kinase due to a heavy work-out: A case report. Cases J. 2008;1(1):173.
26. Thoenes M. Rhabdomyolysis: When Exercising Becomes a Risk. Journal of Pediatric Health Care. 2010;24(3):189-93.
27. Inklebarger J, Galanis N, Kirkos J, Kapetanos G. Exercise-induced rhabdomyolysis from stationary biking: a case report. Hippokratia. 2010;14(4):279.
28. Boni R, Rabitti P. Spinning-induced rhabdomyolysis: importance of MRI for patient’s outcome. A case report. Reumatismo. 2011;63(1):44-8.
29. Krivickas LS. Recurrent rhabdomyolysis in a collegiate athlete: a case report. Med Sci Sports Exerc. 2006;38(3):407-10.
30. Clarkson P. Worst case scenarios: Exertional rhabdomyolysis and acute renal failure. Sports Science Exchange. 1993;4(42):1-6.
31. Knafl EG, Hughes JA, Dimeski G, Eley R. Rhabdomyolysis: Patterns, Circumstances, and Outcomes of Patients Presenting to the Emergency Department. The Ochsner Journal. 2018;18(3):215.
32. Jenkins NT, Martin JS, Laughlin MH, Padilla J. Exercise-induced Signals for Vascular Endothelial Adaptations: Implications for Cardiovascular Disease. Curr Cardiovasc Risk Rep. 2012;6(4):331-46.
33. Pedersen BK. Anti-inflammatory effects of exercise: role in diabetes and cardiovascular disease. Eur J Clin Invest. 2017;47(8):600-11.
34. MacInnis MJ, Gibala MJ. Physiological adaptations to interval training and the role of exercise intensity. J Physiol. 2017;595(9):2915-30.
35. Swain DP. Moderate or vigorous intensity exercise: which is better for improving aerobic fitness? Preventive cardiology. 2005;8(1):55-8.
36. Fealy CE, Nieuwoudt S, Foucher JA, Scelsi AR, Malin SK, Pagadala M, et al. Functional high intensity exercise training ameliorates insulin resistance and cardiometabolic risk factors in type 2 diabetes. Exp Physiol. 2018.
37. Crawford DA, Drake NB, Carper MJ, DeBlauw J, Heinrich KM. Are Changes in Physical Work Capacity Induced by High-Intensity Functional Training Related to Changes in Associated Physiologic Measures? Sports (Basel). 2018;6(2).
38. Feito Y, Hoffstetter W, Serafini P, Mangine G. Changes in body composition, bone metabolism, strength, and skill-specific performance resulting from 16-weeks of HIFT. PLoS One. 2018;13(6):e0198324.